1. Field of the Invention
The present invention relates to a high-frequency composite component, and more particularly, to a high-frequency composite component for use in a plurality of different mobile communication systems.
2. Description of the Related Art
Presently, in Europe, as a mobile communication device, a dual-band portable telephone has been proposed which can operate in a plurality of frequency bands, for example, in the DCS system using a 1.8 GHz band and the GSM system using a 900 MHz band.
FIG. 18 shows a portion of the structure of a general dual-band portable telephone which includes an antenna 1, a diplexer 2, and two signal paths of a DCS system 3 (1.8 GHz band) and a GSM system 4 (900 MHz band).
The diplexer 2 selects a transmission signal from the DCS system 3 or the GSM system 4 in transmission and selects a reception signal to the DCS system 3 or the GSM system 4 in reception. The DCS system 3 includes a high-frequency switch 3a for separating a transmission portion Txd and a reception portion Rxd and a filter 3b for allowing the fundamental frequency of the DCS system to pass through and for attenuating the second and third harmonics. In the same manner, the GSM system 4 also includes a high-frequency switch 4a for separating a transmission portion Txg and a reception portion Rxg and a filter 4b for allowing the fundamental frequency of the GSM system to pass through and for attenuating the third harmonics.
In recent years, a balanced-type (balanced-output type) high-frequency composite component having two signal terminals in the reception portion has been proposed and, in such a balanced type, the impedance matching to a low-noise amplifier (LNA) is required.
In Japanese Unexamined Patent Application Publication No. 2003-142981 (Patent Document 1), as shown in FIG. 19, an inductor 6 is disposed in parallel between the balanced output terminals Rx of a bandpass filter defined by a balanced-output type surface acoustic wave filter. However, it is difficult to set a desired impedance (complex impedance, in particular). According to the knowledge of the present inventor, in order to lower the impedance, a capacitor must be inserted in series to each of the balanced output terminals, and, to increase the impedance, one more inductor must be inserted in parallel between the balanced output terminals in addition to the above-described capacitors. However, when capacitors and inductors as separate components are added between such a high-frequency composite component and an LNA, the number of components and the mounting area increase which increases the size of the equipment, and the matching between the bandpass filter 5 and the LNA becomes more complicated.